One or more embodiments of this invention relates to a heat treated 9 Cr-1 Mo material produced with a 9 Cr-1 Mo alloy of specific composition heat treated in an austenization, rapid cooling, tempering, final cooling cycle, such that the heat treated material exhibits improved resistance to oxidation and high temperature corrosion over currently used high-temperature 9 Cr-1 Mo steels, such as ASTM P91 and ASTM P92, among others. The heat treated 9 Cr-1 Mo material of this invention is suitable for use in situations requiring resistance to high pressures and oxidation resistance at high temperatures, such as steam generators, boilers, chemical industry equipment, and the like.
The constraints placed on power generation in terms of environmental impact and economics have focused attention on the development of high efficiency, low emission systems. Increasing in the thermal efficiency of a power plant is most effectively achieved by increasing the temperature of the steam driving the power-producing turbine. Currently, typical steam power efficiencies are around 42%, with steam temperatures of 600° C. and pressures of 25-30 MPa. Increasing the operating steam temperature to 625-650° C. will enable thermal efficiencies of around 45% to be achieved. However, the increasing operating temperatures and pressures impose increasingly stringent requirements on the materials of construction.
A well-known material capable of satisfying the requirements noted above is austenitic stainless steel. However, austenitic stainless steel is relatively expensive, and its use in commercial plants is limited for economic reasons. In addition, austenitic stainless steel has a large thermal expansion coefficient and can experience relatively large thermal stresses during transient plant operations, start-up, and shutdown. For these reasons, the use of austenitic stainless steel in plants is problematic. More often, 9 Cr-1 Mo steels, such as ASTM P91 and ASTM P92, among others, are used as an effective compromise to balance cost and high-temperature demands.
The 9 Cr-1 Mo steels such as ASTM P91 and ASTM P92, among others, generally provide sufficient strength, resistance to corrosion and oxidation, low thermal expansion, and adequate fatigue resistance. The high chromium (Cr) content in these steels results in an oxide film composed of outer layer iron (Fe) oxides and inner layer Cr oxides or Fe—Cr oxides. Generally, Cr in an amount of not smaller than 8.0% is necessary to form a sound oxide film, while an upper limit of approximately 9.5% is established to allow consistent weldability. Molybdenum (Mo) is used as a solid-solution hardening element and a precipitation-hardening element to form highly dispersed carbides and improve the high temperature creep strength of the steels. Mo is limited to approximately 1% or less, because exposure of the 9 Cr-1 Mo steels with Mo at 600-650° C. has been shown to result in the precipitation of Laves-phase, which removes the element from solid solution and reduces solid-solution strengthening. Additionally, these steels have a typical carbon (C) content of approximately 0.1 wt %, which provides sufficient strength while allowing the material to respond well to hot and cold bending, as well as to welding. The stress rupture strengths of these steels are increased by the addition of carbide formers Niobium (Nb) and Vanadium (V). Tungsten (W) is further added to ASTM P92 to allow operations at slightly higher temperatures than P91, but at increased cost. However, in the currently sought temperature environment of 625-650° C., none of the currently used high-temperature steels such as ASTM P91 and ASTM P92, among others, have a satisfactory level of resistance to oxidation and corrosion, and typically the highest service temperature achievable is limited to 625° C.
The resistance to oxidation and corrosion at higher temperatures can be achieved by increasing the content of Cr to improve oxidation resistance, and adding nickel (Ni) to suppress any resulting δ-ferrite, however a high alloy steel with a high content of Cr and Ni significantly increases cost and becomes comparable to an 18-8 austenitic stainless steel from an economic standpoint. Similarly, cobalt (Co) can be utilized to improve the performance of 9 Cr-1Mo steels at higher temperature, but like W and Ni, the addition of Co can be unattractive economically. It would be advantageous to produce a material similar in composition to commonly used high-temperature steels such as ASTM P91 and ASTM P92 that utilizes a relatively inexpensive alloying addition for increased high-temperature performance.
Titanium (Ti) is an economically attractive alloying element and has been investigated for 9 Cr-1 Mo steels. Typically, Ti has been added as a stabilizer preventing sensitization for applications where high strength requirements limit the degree to which C can be reduced. This practice exploits the stronger tendency of Ti over Cr to form carbides, thus permitting the matrix to retain the corrosion inhibiting Cr. However, it is known that Ti can impart brittleness, and the use of Ti as a stabilizer typically emphasizes a Ti content as low as possible, but at a ratio to C or C plus nitrogen (N) on the order of ten or more. See Grubb, et al, “Micromechanisms of Brittle Fracture in Titanium-stabilized and {acute over (α)}-Embrittled Ferritic Stainless Steels,” Toughness of Ferritic Stainless Steels, American Society of Testing and Materials STP 706 (1980). This combination of requirements tends to necessitate a relatively low carbon level of typically 0.03% or less when Ti stabilization is utilized, which limits application where higher strengths and hardness are required. See U.S. Pat. No. 5,851,316, issued to Yazawa, et al, issued Dec. 22, 1998; U.S. Pat. No. 5,843,370, issued to Koyama, et al, issued Dec. 1, 1998; U.S. Pat. No. 5,051,234, issued to Shinagawa, et al, issued Sep. 24, 1991; U.S. Pat. No. 4,640,722, issued to Gorman, issued Feb. 3, 1987; U.S. Pat. No. 4,461,811, issued to Borneman, et al, issued Jul. 24, 1984; U.S. Pat. No. 4,261,739, issued to Douthett, et al, issued Apr. 14, 1981; U.S. Pat. No. 3,953,201, issued to Wood, et al, issued Apr. 27, 1976. Ti and Nb have also been used in combination for stabilization, but low carbon levels remain a requirement. Additionally, Mo is often treated as an optional or impurity element. See U.S. Pat. No. 4,964,926, issued to Hill, issued Oct. 23, 1990; U.S. Pat. No. 4,834,808, issued to Hill, issued May 30, 1989; U.S. Pat. No. 4,581,066, issued to Maruhashi, et al, issued Apr. 8, 1986.
Ti has also been utilized in 9 Cr-1 Mo steels as a carbide-forming agent which contributes to precipitation strengthening. Precipitation strengthening with Ti requires the dissolution of primary titanium carbides by austenization at high temperature, often greater than 1300° C., in order to dissolve the low-solubility primary titanium carbide as completely as possible. On reheating, fine precipitates of secondary titanium carbide typically less than 30 nm in size distribute throughout the matrix and provide strengthening by acting to impede the movement of dislocations. Dissolution of all or most of the primary titanium carbide during austenization is usually specified, and remaining primary titanium carbides are strictly minimized to avoid degradation of creep properties. Hot working in the austenite temperature range can also be specified to further promote the dissolution of the primary titanium carbides. The latter step, in particular, adds significant processing time and cost to a typical heat treatment that might otherwise consist solely of austenization, cooling, and tempering.
As an example, U.S. Pat. No. 5,310,431, issued to Buck, issued on May 10, 1994, discusses a steel utilizing Ti as an alloying element for precipitation strengthening in a steel similar in composition to commonly used high-temperature 9 Cr-1 Mo steels, such as ASTM P91 or ASTM P92. The '431 steel produces secondary MX precipitates (M=Ti, Nb, Hafnium (Hf), Zirconium (Zr), and Tantalum (Ta), and X=C) to reduce interparticle spacing and produce a microstructure of uniformly dispersed MX precipitates. A stated primary objective during austenization of the '431 steel is dissolution of all or most of the primary MX precipitates, as any remaining primary MX precipitates following austenization are said to reduce the creep properties and toughness of the '431 steel. The '431 steel specifies an alloy which is face-centered cubic (FCC) above 900° C. and extensively discusses stabilizing elements to avoid formation of any body centered cubic (BCC) high temperature δ-ferrite during austenization. Acceptable austenite stabilizers are Co, Copper (Cu), or Zr, with Co as the preferred element. The '431 patent presents examples of a Ti bearing alloy austenized at 1300° C. in order to form secondary TiC precipitates, but in order to avoid the formation of δ-ferrite and maintain the specified FCC structure at this temperature, a steel with base composition of 3.0 wt. % Co and 1.0 wt. % Ni is used. A typically used high-temperature 9 Cr-1 Mo steel such as ASTM P91 or ASTM P92, which minimizes Ni content and avoids Co as an alloying element for economic reasons, would not avoid the formation of δ-ferrite if alloyed with Ti and austenized at 1300° C. Additionally, the '431 steel provokes the precipitation of secondary MX precipitates by cooling the alloy to a temperature above ambient, preferably 900° C., and holding for ½ hour before allowing the alloy to cool to room temperature. This stepped cooling method produces a microstructure of martensitic, bainitic, and ferritic steel.
U.S. patent application Ser. No. 11/250,492, submitted by Fujitsuna, et al, published Mar. 16, 2006, also discusses a steel utilizing Ti as an optional alloying element to form secondary titanium carbides in a steel similar in composition to commonly used high-temperature 9 Cr-1 Mo steels, such as ASTM P91 or ASTM P92. The '492 steel specifies dissolution of primary titanium carbides by extensive predetermined plastic working, such as forging, rolling, extrusion, or the like, at temperatures preferably 1300° C. or higher. Following this hot working, the '492 steel is annealed at a temperature between 1000° C. and 1150° C. for one hour, cooled below its Ac1 transformation temperature, and tempered at a temperature between 650° C. and 800° C. for one hour. This process is designed to produce steel having a martensitic structure with no large or coarse primary titanium carbides. The '492 application does demonstrate a titanium alloyed steel exhibiting slightly increased creep rupture performance over comparison steels similar in composition to ASTM P91 or ASTM P92 after this treatment, however, as earlier stated, the requirement for extensive hot working at temperatures of 1250° C. or greater adds significant processing time and cost. Additionally, the titanium alloyed steel demonstrated by the '492 application also contains tungsten which, like cobalt, is a relatively expensive alloying agent to be avoided if possible.
U.S. Pat. No. 6,514,359, issued to Kawano, issued Feb. 4, 2003, also discusses a steel utilizing Ti as an optional alloying element to form secondary titanium carbides in a steel similar in composition to commonly used high-temperature 9 Cr-1 Mo steels such as ASTM P91 or ASTM P92. The '359 patent considers V containing steels with Nb, Ti, N, Ta, Cu, Ni, and/or Co added to form primary MX precipitates, and calls for dissolution of these primary MX precipitates at an austenization temperature preferably 900-1100° C. The dissolved primary MX precipitates subsequently form precipitation strengthening secondary MX precipitates 30 nm or less at tempering. The '359 patent demonstrates one example steel wherein Nb and Ti are utilized as the precipitation strengthening elements, and indicates that secondary MX precipitations occur following the specified heat treatment with austenization at 950° C. However, given the extraordinarily low solubility of primary titanium carbide in ferrite at 950° C., it is unlikely that primary titanium carbide is a significant contributor to these particular secondary MX precipitates.
The object of one or more embodiments disclosed herein is to provide a heat treated 9Cr-1Mo material produced with a 9 Cr-1 Mo alloy of specific composition that is heat treated in an austenization, rapid cooling, tempering, final cooling cycle, such that titanium carbides exist in the material as both primary and secondary precipitates. This heat treated 9Cr-1 Mo material exhibits improved high-temperature creep strength and improved oxidation and corrosion resistance in a temperature environment of 625-650° C. The heat treated 9Cr-1Mo material is produced through a novel combination of composition and heat treatment whereby relatively large (0.5-3 μm) primary titanium carbides are formed during steel production prior to the heat treatment. During conduct of the heat treatment, 40-60% of the primary titanium carbides are dissolved into Ti and C, which subsequently precipitate as relatively small (5-50 nm) secondary titanium carbide precipitates. Unlike many of the existing Ti alloyed 9 Cr-1 Mo materials, which emphasize hot working requirements intended to minimize primary titanium carbide and maximize secondary titanium carbide, this process produces a heat treated 9Cr-1Mo material which intentionally retains a percentage of the primary titanium carbides for creep strength in conjunction with precipitated secondary titanium carbides distributed throughout the matrix. The secondary titanium carbides maintain a higher level of chromium in the finished steel for increased oxidation resistance, and strengthen the steel by impeding the movement of dislocations through the crystal structure.
The heat treated 9Cr-1Mo material thereby provides substantial performance and economic advantage on several fronts. First, the material uses an additive alloying element, Ti, that is relatively inexpensive as compared to W, Ni, Co, or other alloying element additions, to produce a material comparable in cost to currently used high-temperature 9 Cr-1 Mo steels, such as ASTM P91 and ASTM P92, among others. Second, the heat treated 9Cr-1Mo material requires only an austenization, rapid cooling, tempering, final cooling cycle to realize substantial performance improvements, and avoids costly and time-consuming requirements for hot-working in the austenite temperature range. Third, the improved high-temperature properties significantly reduce the frequency of costly operational shutdowns for necessary replacement of fabricated items. Additional advantages of the heat treated 9Cr-1Mo material not listed here will undoubtedly further accrue due to its improved performance over currently used high-temperature 9 Cr-1 Mo steels available at comparable cost.